docs: def nearest_keys(self, key):
          """Find the nearest_keys (l2 distance) thanks to a cKDTree query"""
          if not isinstance(key, tuple):
              _key = (key,)
          if self.__stale:
              self.generate_tree()
          d, idx = self.__tree.query(
              _key, self.k_neighbors, distance_upper_bound=self.distance_upper_bound)
  
          try:
              return [self.__keys[id][0] for id in idx if id < len(self.__keys)]
          except TypeError:
              # if k_neighbors = 1 query is not returnng arrays
              return self.__keys[idx]